1. Field of the Invention
The present invention relates to a navigation system that corrects a measured position obtained by using a GPS receiver, and that uses the corrected measured position.
2. Description of the Related Art
It has been conventionally known that a measured position obtained by using a GPS receiver varies due to error factors, such as multipaths, and a navigation system that corrects the measured position so as to reduce an influence caused by the error is known (refer to, e.g., Japanese Unexamined Patent Application Publication No. 8-313278, pages 5 to 8 and FIGS. 1 to 3). The navigation system estimates the current measured position based on the previous measured position to set an estimated error range. When the current measured position is in the estimated error range, the navigation system uses the actual measured position, and when the current measured position is out of the estimated error range, the navigation system uses the estimated measured position. Thus, when error in the measured position using GPS is large, the navigation system can reduce the influence.
In the identified Publication, when the current measured position is out of the estimated error range, the estimated measured position is used. Since the estimated error range is set centering at the estimated measured position, the estimated measured position is inevitably set at the center of the estimated error range. Thus, for example, even when the estimated measured position is slightly out of the estimated error range, the estimated measured position is used without considering the slight difference. Thus, there is a problem in that error is increased. The Publication also discloses a method for correcting the measured position by adding the estimated measured position and the actual measured position through predetermined weighting. In this method, even when the measured position is expected to be in the estimated error range, the corrected measured position may be out of the estimated error range (in particular, when the actual measured position is greatly out of the estimated error range). Thus, there is a problem in that error can be increased. For example, since it is considered that the accuracies of a velocity and a travel direction detected with a GPS receiver are high, the possibility that an error-free measured position is in the estimated error range is considerably increased when the current measured position is assumed to be correct.